The invention relates generally to osmotic pumps for delivering beneficial agents. More specifically, the invention relates to an osmotic pump having a membrane plug for controlling the delivery rate of a beneficial agent.
Osmotic pumps for delivering beneficial agents within the body of a patient are known in the art. For illustration purposes, FIG. 1A shows a cross-section of a prior-art osmotic pump 100 having an implantable capsule 102 with open ends 104, 106. A diffusion moderator (also called flow modulator) 108 is disposed in the open end 106 of the capsule 102. The diffusion moderator 108 has a delivery path 110 that terminates at a delivery port 112 and allows fluid from the interior of the capsule 102 to be transported to the exterior of the capsule 102. A membrane plug 114 is inserted in the open end 104 of the capsule 102. The membrane plug 114 is made of a semipermeable material and forms a fluid-permeable barrier between the exterior and the interior of the capsule 102. A piston 116 is disposed in the capsule 102. The piston 116 defines two chambers 118, 120 within the capsule 102. The chamber 118 contains an osmotic agent 122, and the chamber 120 contains a beneficial agent 124.
When the osmotic pump 100 is implanted in a patient, fluid from the body of the patient enters the chamber 118 through the membrane plug 114, permeates the osmotic agent 122, and causes the osmotic agent 122 to swell. The swollen osmotic agent 122 pushes the piston 116 in a direction away from the membrane plug 114, reducing the volume of the chamber 120 and forcing an amount of the beneficial agent 124 out of the capsule 102 through the diffusion moderator 108 into the body of the patient. The rate at which the osmotic pump 100 delivers the beneficial agent 124 to the body depends on the rate at which fluid permeates the membrane plug 114.
Typically, the membrane plug 114 is made of a hydratable compound that must hydrate in order for the osmotic agent 122 to begin absorbing moisture. The time to hydrate the membrane plug 114 and the osmotic agent 122 delays the start of ejection of the beneficial agent 124 from the chamber 120. During this startup phase, body fluid, usually water, can back-diffuse into the delivery port 112 of the diffusion moderator 108 and degrade the beneficial agent 124 or the vehicle carrying the beneficial agent 124. Some vehicles when they combine with water can plug the delivery path 110.
If the delivery path 110 or port 112 becomes plugged, for example, due to a lengthy startup, or if the piston 116 becomes stuck inside the capsule 102, there will be pressure buildup in the chamber 118, which may be sufficient to expel the membrane plug 114 from the capsule 102.
Various methods have been proposed for avoiding expulsion of the membrane plug 114 from the capsule 102. One method involves securing the membrane plug 114 to the capsule 102 using an adhesive. This method requires an additional operation to apply the adhesive to the membrane plug 114 and/or the capsule 102, and the adhesive may affect the permeability of the membrane plug 114. Another method for avoiding expulsion of the membrane plug 114 is to drill a hole in the end portion of the capsule 102 containing the membrane plug 114. FIG. 1B shows a hole 126 drilled in the capsule 102. As shown in FIG. 1B, the hole 126 is initially covered by the membrane plug 114, but as the membrane plug 114 is forced out of the capsule 102 due to pressure buildup in the chamber 118, the hole 126 will eventually become exposed, allowing pressure to be vented from the chamber 118 to the exterior of the capsule 102. In this manner, the membrane plug 114 is prevented from becoming separated from the capsule 102. This method requires an additional operation in the fabrication of the capsule 102 and increases the overall cost of the osmotic pump.